1. Field of the Invention
The present invention relates to miniaturized oxygen electrodes, i.e., Clark-type "micro-" oxygen electrodes, and processes for the production thereof using a semiconductor fabrication technology. The oxygen electrodes can be advantageously used for determining a concentration of oxygen dissolved in a specific solution or other mediums. For example, these electrodes can be used as a device for measuring BOD (Biological Oxygen Demand) in water in the field of water control and the like. Further, in fermentation industries, such electrodes can be used to determine the concentration of dissolved oxygen in a fermentation tank, to realize an effective alcohol fermentation therein. Furthermore, such electrodes can be used as a transducer to produce enzyme electrodes or biosensors for, for example, sugars and vitamins. As an example, such a biosensor, when combined with the enzyme: GOD (glucose oxidase) as a catalyst, can act as a glucose sensor. This is because glucose (C.sub.6 H.sub.6 O.sub.12) is oxidized to gluconolactone (C.sub.6 H.sub.10 O.sub.6), when reacted with dissolved oxygen in the presence of the catalyst BOD, and as a result of this oxidation reaction, the amount of the dissolved oxygen diffused into a sensing cell of the oxygen electrode is reduced. Based on this reduction of the dissolved oxygen, a concentration of glucose can be exactly determined. The micro-oxygen electrodes of the present invention can be widely utilized in the fields of, for example, clinical analysis, industrial processing, and environmental conditioning.
The present invention also relates to miniaturized biosensors i.e., "micro-" biosensors. These micro-biosensors also can be widely used in various fields, similar to the micro-oxygen electrodes, since these sensors are extremely small and are disposable, if desired, due to low production costs. For example, in the medical and clinical fields, the micro-biosensors can be combined with a catheter to carry out in vivo measurements.
2. Description of the Related Art
As well known in the art, there are two groups of Clark-type oxygen electrodes. Namely, polarographic electrodes in which a determination of the oxygen concentration is carried out by applying a predetermined voltage between the electrodes, and galvanic electrodes wherein the oxygen determination is made by utilizing spontaneously proceeding reactions. These oxygen electrodes have similar structures, but are distinguishable by the materials used for the electrode structure. If both the cathodes and the anodes are made from chemically stable noble metals such as gold and platinum, they are classified as polarographic oxygen electrodes, but if the anodes are made from metals having a higher tendency to induce chemical reactions than the noble metals, for example, lead and silver, they are classified as galvanic oxygen electrodes.
A typical example of the prior art Clark-type oxygen electrodes is illustrated in FIG. 1. The illustrated electrode is a galvanic oxygen electrode and is known and conveniently used as an oximeter for measuring dissolved oxygen. The electrode consists of an open-ended glass container 21, an oxygen gas-permeable membrane 25 such as TEFLON (polytetrafluoroethylene) covering a bottom portion of the container 21 and sealed with an O-ring 22, an electrolyte solution 24 such as 1 M KOH retained in the container 21 and two electrodes, i.e., a working electrode (cathode) 23A made of e.g., platinum, and a counter electrode (anode) 23B made of, e.g., lead. Details concerning the structure and determination of the oxygen concentration of the Clark-type electrode can be found in many references, such as S. Suzuki, "Ion electrodes and enzyme electrodes", 1981, Kodansha, Tokyo.
The prior art Clark-type oxygen electrodes, however, are not suitable for mass-production because they must be manually fabricated using a glass working technology, and, are thus very expensive. They are also unacceptably large for many usages; e.g., they cannot be used for in vivo measurements of oxygen concentration. Moreover for technical reasons, they cannot be miniaturized to a size smaller than that of, for example, a pencil.
New Clark-type oxygen electrodes fabricated by using a semiconductor fabrication technology in which the drawbacks of the glass-made electrodes are avoided, have been developed by Prof. T. Moriizumi et al. of the Tokyo Institute of Technology. As reported in, for example, Y. Miyahara, F. Matsu, S. Shiokawa, T. Moriizumi, H. Matsuoka, I. Karube and S. Suzuki, Proc. of the 3rd Sensor Symp., (Inst. Electr. Eng. Jap.), 21 (1983), the miniaturized and integrated oxygen electrodes are produced by anisotropically etching a silicon wafer to form V-shaped grooves on a surface of the wafer, depositing the Au cathode and Ag anode on a selected surface of the wafer, pouring an alkaline solution of electrolyte into the grooves, and finally, covering the electrolyte solution-containing grooves with an oxygen-permeable membrane, made of, e.g., TEFLON. The thus-produced oxygen electrode is shown as a cross-sectional view of the sensing site of the electrode in FIG. 2 of the accompanying drawings.
In FIG. 2, the oxygen electrode consists of a silicon chip 31 having a V-shaped groove formed on a surface thereof. Two silver (Ag) electrodes 32 and a gold (Au) electrode 33 are deposited on the V-grooves and are covered with a TEFLON membrane 30. A space formed between the V-grooves and the membrane 30 is filled with an aqueous solution of an electrolyte 36 such as NaOH or NaCl. The resulting sensing site of the electrode is placed in contact with another silicon chip 34, and sealing using an epoxy adhesive 38 is carried out. Reference numerals 35 and 37 each represent a silicon oxide coating obtained through a thermal oxidation of the exposed silicon chips 34 and 31, respectively. This oxygen electrode has many advantages, for example, extremely small size, reduction of the necessary amount of samples to be tested, high reliability and precision, and mass-production capability, compared with the above glass electrodes, but suffer from several disadvantages. For example, since the TEFLON coating used as the gas-permeable membrane will not adhere to many materials, it is necessary to use an additional adhesive means to assist the adhesion of this coating. Further, the structure of the electrode is relatively complicated, and it is desirable to provide a more simplified electrode structure.
The oxygen electrode of Japanese Unexamined Patent Publication (Kokai) No. 60-146145 was invented to solve the problems of the oxygen electrode described above with reference to FIG. 2. From FIG. 3 of the accompanying drawings, it can be understood that this electrode is similar to that of FIG. 2, except that an anode electrode was formed on a silicon (Si) substrate, and a cathode electrode was formed on an electrolyte-facing surface of the gas-permeable membrane, respectively. In FIG. 3, 30 is a gas-permeable membrane made of, e.g., TEFLON, 31 a lower Si wafer, 32 a Cr-Ag anode, 33 an Au cathode, 34 an upper Si wafer, 35 an SiO.sub.2 layer, 36 an electrolyte solution such as 1 M KOH, 37 an SiO.sub.2 layer, and 38 an epoxy sealing agent. According to this electrode, the steps necessary to produce the electrode can be reduced and a response speed of the electrode can be increased.
Also, the oxygen electrode of Japanese Unexamined Patent Publication (Kokai) No. 61-30756 was invented to solve the problems of the electrode of FIG. 2. The oxidation electrode of this reference, as illustrated in FIG. 4, has a structure similar to that of FIG. 2 and comprises a silicon chip 31, an Ag anode 32, an Au cathode 33, and an SiO.sub.2 layer 37. Recesses on the chip 31 contain an electrolyte solution 36 such as NaOH or NaCl. An upper surface of the chip 31 including recesses is covered with an organic coating 30 such as polyamide. In order to adhere the coating 30 to the silicon chip 31 and the cathode 33, an interlayer 39 such as a polyaminosiloxane coating is used. The organic coating 30 and the interlayer 39, make it possible to simplify the complicated fabrication steps in the production of the miniaturized and integrated oxygen electrodes.
In the production of the prior art miniaturized oxygen electrodes discussed above, the anisotropic etching technology was used to form microrecesses on the silicon wafer, into which the electrolyte solution is then poured. However, since this technology makes the process complex and cumbersome, and requires the use of hydrofluoric acid which is dangerous to operators, it is desirable to develop an improved production process of the miniaturized oxygen electrodes without using the conventional anisotropic etching technology.
Recently, such a production process was invented by researchers at Fujitsu Limited (see Japanese Unexamined Patent Publication (Kokai) No. 62-39755). As apparent from FIG. 5 of the accompanying drawings, the production process comprises patterning an Ag anode 42 on a surface of a glass substrate 41, coating a photosensitive resin 44 on a surface of the substrate 41, selectively etching the resin coating 44 to form a cell 45 and an output terminal 46 of the anode 42, and then patterning an Au cathode 43 on the surface of the resin coating 44. The electrolyte solution is poured through an injection port 47 into the cell 45. This production process makes it possible to safely and easily produce miniaturized oxygen electrodes without disconnection of the electrodes, and in addition, the anisotropic etching step can be omitted.
Generally speaking, the prior art oxygen electrodes are considered to be satisfactory, since they are compact and have a simple structure. However, undesirably, the electrolyte solution used tends to have an adverse effect on the resulting electrode, since it is in the form of liquid. Further, the gas-permeable membrane, particularly the coating of fluoropolymers, causes problems due to poor adhesion properties. It is, therefore, desired to provide a further improved miniaturized oxygen electrode and a production process thereof, as well as an improved miniaturized biosensor.